In terms of the site itself, I would want Du to be able to change your username. Some sites only allow Du to do it every 6 months after Du last changed it, so even if Fanpop would just do that, I'd be a lot happier with the site. And if there was an app, that'd be awesome!
The only thing I'd like to really see on Fanpop is Mehr Auszeichnungen and Mehr opportunities for Du to get to talk with the admins of the site, like Dave and Cliff.